1. Field of the Invention
The present invention relates to an X-ray CT apparatus in which a conical X-ray beam is radiated on a region of an object to obtain optional tomogram of the part.
2. Prior Art
For obtaining a tomogram of a target region of an object (patient and so on) by means of a conical beam type X-ray CT (computed Tomography) apparatus, a local radiography is executed such that the target region is extremely limited. Therefore, it is required to determine the target region in advance in order to accurately include the target region to be imaged in the projection area. In the prior art the target region has been determined by the following method.
At first, CT imaging is executed, and then plural fluoroscopic images for specifying a reconstruction position, namely for specifying a target region, are obtained in order to reconstruct a tomogram (CT image). A desired fluoroscopic image is picked up among the obtained plural fluoroscopic images and a three-dimensional position of the target region is determined based on the fluoroscopic image that has been picked up. Then, the region around the determined position is reconstructed, thereby obtaining a tomogram of the target region. Such an X-ray CT method is disclosed in JP-A-2001-292991.
According to such a prior art, the tomogram of a relatively large area such that the target region is included in the projection area is obtained, and then the target region is found out among the fluoroscopic images which have been obtained by CT scan on a large area, and the position to be reconstructed as a tomogram is obtained.
In this method, because the imaging apparatus for a large area is required and the fluoroscopic images in a large area are required to be obtained, the X-ray exposed dose is tends to be increased and the X-ray CT apparatus becomes to be large scaled. Therefore, there is a room for improvement in downsizing the apparatus and in expediting imaging by reducing the time required for obtaining tomograms.